1. Field of the Invention
The present invention relates to a switching regulator capable of preventing variations in the output voltage and operating by consuming low power, under light load conditions.
2. Description of the Related Art
A conventional switching regulator performs PWM (Pulse Width Modulation) control under regular conditions, and switches to performing VFM (Variable Frequency Modulation) control under light load conditions.
With PWM control, the constant voltage output operation is performed by fixing the frame frequency and feeding back a monitored output status to adjust the time during which a switch element is turned on in accordance with the fed-back status. Meanwhile, in VFM control, the constant voltage output operation is performed by fixing the time during which the switch element is turned on, or fixing the minimum time during which the switch element is turned on, and feeding back a monitored output status to adjust the frame frequency in accordance with the fed-back status. VFM control is more advantageous than PWM control in terms of achieving high efficiency under light load conditions. This is because in PWM control, the switch element is turned on at every predetermined frequency even under light load conditions, whereas in VFM control, the number of times the switch element is turned on/off can be reduced.
For example, there is a DC-DC converter capable of operating with low power consumption under light load conditions, by controlling the timing at which the switch element is turned on, such that a division voltage obtained by dividing the output voltage becomes a predetermined reference voltage.
Patent Document 1: Japanese Laid-Open Patent Application No. H10-225105
However, the problem with such a DC-DC converter is that the timing of turning on the switch element, i.e., the timing of switching to VFM control, is determined based on an error voltage obtained by amplifying the voltage difference between the division voltage and the reference voltage, and also based on a previously specified reference voltage. Accordingly, this switching operation cannot be performed with an input signal from outside. Thus, this technology cannot be applied for operating the switching regulator with a fixed frequency. Moreover, the optimum value of the second reference voltage, which determines the timing of switching between VFM control and PWM control, may vary depending on the level of the input voltage. For this reason, it has been difficult to improve the efficiency under light load conditions and to reduce the ripple voltage, in accordance with a wide range of input voltage levels.